Humidity-Dependent Microviscosity of Submicron Respiratory Aerosols Probed via Molecular Rotors

Respiratory viruses such as influenza and SARS-CoV-2 can be transmitted through aerosols and respiratory droplets, contributing substantially to public health burdens. Environmental factors like relative humidity (RH) have been shown to affect transmission rates, motivating studies into the aerosol physicochemical properties that can affect virus viability. However, small particle sizes and low mass densities in air have made in situ measurements of the viscosity of submicron aerosols challenging. Here, we have modified the recently developed fluorescence aerosol flow tube and used the molecular rotor trans-4-[4-(dimethylamino)styryl]-1-methylpyridinium iodide in conjunction with time-correlated single-photon counting to characterize the viscosity of submicron model respiratory aerosols in situ. A calibration curve was constructed using samples of known viscosity, showing that the fluorophore was sensitive over a large viscosity range from 10–2 Pa·s up to response saturation around approximately 106–109 Pa·s. Subsequently, the RH-dependent viscosity was measured in three main respiratory aerosol systems: artificial saliva, Dulbecco’s modified Eagle’s medium, and simulated lung fluid. Each model system exhibited the same qualitative pattern of higher viscosities at low RH and lower viscosities at high RH, with specific quantitative differences attributable to the chemical compositions and phase states of the different particle types. Looking forward, this study demonstrates the power of fluorescence-based approaches to characterize viscosity in situ in submicron aerosol particles across a large viscosity range and at relatively low particle concentrations.